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A composite foundation in the current study is improved by the joint usage of two types of columns.The soil is modeled as a Mohr-Coulomb material,and the stress-strain relationship is simplified as bilinear elastio-plastic with associated flow rule.Explicit expressions of the equivalent modulus of the composite foundation under elastic or/and plastic conditions are obtained using the parametric variational principle.A critical stress is proposed to judge whether plastic state of soils occurs under a prescribed vertical loading.As a result,the equivalent modulus,taking into account the spatial variation of soil parameters,can improve the prediction in settlement of the composite foundation.The geostatistical technique is recommended to evaluate the correlation length in soils along the vertical direction.Considering the relatively complex mathematical formula in the limit state equation,Monte-Carlo simulation is employed to calculate the probability failure.Finally,a reliability-based design methodology is proposed to balance the safety and economy considerations in designing the area replacement ratios of those two types of columns in a composite foundation.